Both u and v must be 0-d or 1-d (ndim = 0 or 1), but u.ndim need
not equal v.ndim. In other words, all four possible combinations -
u.ndim=v.ndim=0, u.ndim=v.ndim=1,
u.ndim=1,v.ndim=0, and u.ndim=0,v.ndim=1 - work.